FIG. 12 shows an example of a conventional type auto-drain unit, in which an auto-drain unit 1 comprises a bowl 2 for storing drain coming through a drain inlet (not shown), a cylindrical valve seat member 3 engaged in a drain outlet 2b at the inner bottom of the bowl 2, a drain valve seat 4 having a sealing member mounted on the valve seat member 3, a drain valve-disc 5 for opening and closing the drain valve seat 4, and a piston 6 integrally constructed with the drain valve disc 5.
Said piston 6 is accommodated in a cover 8 above the valve seat member 3 in such manner that the piston is moved up and down, and it is permanently pushed downward by a return spring 12, i.e. in a direction to open the drain valve seat 4 by the drain valve disc 5. On the cover 8, a guide tube 9 for guiding upward and downward movement of a float 10 is provided.
On the upper surface of the piston 6, a pilot chamber 13 is formed, and the pilot chamber 13 is communicated with the bowl via a pilot valve seat 15 on the upper end of the guide tube 9 and is also permanently communicated with the drain outlet 2b via an orifice 19 having a diameter smaller than that of the pilot valve seat 15. On the lower surface of the piston 6, a pressure receiving chamber 14 directly communicated with the bowl 2 is provided.
A pilot valve disc 16 for opening and closing the pilot valve seat 15 is mounted on a rotating member 17, which is tilted by upward or downward movement of the float 10. When the float 10 is moved down, the pilot valve seat 15 is closed, and when the float 10 is moved up to a predetermined position, the pilot valve seat 15 is opened.
In the conventional type auto-drain unit 1 as described above, when liquid level of the drain in the bowl 2 is low, the float 10 is moved down and the pilot valve disc 16 closes the pilot valve seat 15. Thus, air pressure in the pilot chamber 13 opened to the atmospheric air via the orifice 19 is lower than the pressure in the pressure receiving chamber 14, which is communicated with the bowl 2. As a result, the piston 6 is moved to a position higher than the position in the figure against resilient force of the return spring 12, and the drain valve disc 5 closes the drain valve seat 4.
When liquid level of the drain is high and the float 10 is moved up to a predetermined height, the rotating member 17 is rotated counterclockwise, and the pilot valve disc 16 opens the pilot valve seat 15. Thus, pressurized air in the bowl 2 enters the pilot chamber 13. As a result, the sum of the operating force caused by air pressure, which is applied downward on the piston 6 in the pilot chamber 13, and the operating force caused by the return spring 12 becomes higher than the operating force applied upward on the piston 6 by drain in the pressure receiving chamber 14, and the piston 6 is moved down. Then, the drain valve disc 5 opens the drain valve seat 4 to discharge drain.
When the liquid level of the drain is decreased, the float 10 moves down, and the rotating member 17 is rotated clockwise, and the pilot valve disc 16 closes the pilot valve seat 15. Thus, the air in the pilot chamber 13 is gradually discharged to the outside through the smaller diameter orifice 19, and the air pressure decreases. Then, the piston 6 is moved up, and the drain valve disc 5 closes the drain valve seat 4, completing the discharge of the drain.
In the conventional type auto-drain unit 1, many members including the piston 6 are immersed in the drain, which is stored in the bowl 2. As a result, lubricant on the sealing member is washed off by drain, or foreign objects (such as solid matters, slurry, etc.) in the drain are caught. These adverse effects of the drain often cause erroneous operation and hinders maintenance operation to correct trouble or defective operation.
Because pilot air in the pilot chamber 13 is discharged through the drain outlet 2b together with the drain, the pilot air is not smoothly discharged in case a long and highly resistant pipe is connected to the drain outlet 2b, and erroneous operation may occur.
Further, manual operation from outside cannot be achieved, and it is also difficult to design such manual operation means.